Solar hydrogen generation: toward a renewable energy future

Solar Hydrogen Generation

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Solar Hydrogen Generation
Toward a Renewable Energy Future

Edited by

Krishnan Rajeshwar
University of Texas at Arlington, TX, USA

Robert McConnell
Amonix, Inc., Torrance, CA, USA

Stuart Licht
University of Massachusetts, Boston, USA

123
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Editors
Krishnan Rajeshwar
Department of Chemistry & Biochemistry
University of Texas, Arlington
Arlington TX 76019-0065
USA

Robert McConnell
Amonix, Inc.
3425 Fujita St.
Torrance, CA 90505


Stuart Licht
Chemistry Division
National Science Foundation
4201 Wilson Blvd.
Arlington, VA 022230
USA

Department of Chemistry
University of Massachusetts, Boston
100 Morrissey Blvd.
Boston MA 02135
USA


ISBN: 978-0-387-72809-4

e-ISBN: 978-0-387-72810-0

Library of Congress Control Number: 2007943478
c 2008 Springer Science+Business Media, LLC
All rights reserved. This work may not be translated or copied in whole or in part without the written
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Dedication

Krishnan Rajeshwar
To the three girls in my life, Rohini, Reena, and Rebecca: I could not have done this
without your love and support

Robert McConnell
To my wife Suzie Star whose love and support made this possible. To my children and
especially my grandson Tharyn. My hope for them is to live in a cleaner
world powered by renewable energy and hydrogen.

Stuart Licht
To my children: Reeva, Gadi, Ariel, Jacob and Dov; I hope to open a
path to a sustainable energy future for them. To my wife Bregt, this is here
because you are here.

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Preface

This book examines ways to generate hydrogen from sunlight and water. It largely
arose out of a desire to bring all the disparate ways to accomplish this goal within the
confines of a single edited volume. Thus we are aware of many books and reports
discussing the pros and cons of a hydrogen economy but none, that we are aware of,
that focus on the science and technology of generating hydrogen from sunlight and
water. While renewable hydrogen currently remains an elusive goal, at least from a
cost perspective, the scientific principles behind its generation are well understood.
Thus over and above reviewing this substantial fundamental database, part of the
incentive for creating this book was to hopefully inspire future generations of
scientists and engineers to respond to the grand challenge of translating the
impressive laboratory advances and prototype demonstrations to a practical
renewable energy economy. Much of this daunting hurdle has to do with optimizing
the efficiency and hence the cost-effectiveness of hydrogen producing solar energy
systems.
History certainly is on our side in meeting this challenge. Many early
civilizations used the sun, water, and the wind to meet basic needs. Even geothermal
heat was used by North American Indians some 10,000 years ago for cooking. The
ancient Greeks used hydro power to grind flour and the Persians used windmills to
pump water in the first millennium. The human race is very good at solving
technological problems and we can certainly wean ourselves from fossil fuels if we
collectively put our minds to it. But cost is certainly going to be a driver and no
amount of civic sense is going to render the hydrogen economy practically realizable
if a gallon of gasoline continues to be substantially cheaper than a kilogram of
hydrogen. Unfortunately however we can only give short shrift to the issue of
economics in this book because of the rapidly shifting landscape of assumptions that
an evolving technology brings with it. Nonetheless, the concluding chapter of this
book examines investments, levelized hydrogen prices, and fuel cycle greenhouse

gas emissions of a centralized electrolytic hydrogen production and distribution
system powered by photovoltaic electricity.

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viii

Preface

Another important and related topic, not specifically addressed in this book,
concerns the issue of how to store hydrogen, especially in a mobile transportation
application. We felt that this topic was specialized and wide ranging enough to
warrant a separate volume to be created by scientists and engineers far more
qualified and knowledgeable than us. While fuel cells are briefly introduced in
Chapter 1, how hydrogen is to be utilized to generate power is again left to many
other excellent treatises in the literature; some of these are cited in what follows.
Every effort was made to remove redundancy and add homogeneity to the
material in this multi-author volume. Indeed, the more authoritative level of
discussion afforded by having specialists write each chapter will have hopefully
overridden any “rough edges” that remain from chapter to chapter. Undoubtedly,
many flaws remain for which we as editors are wholly responsible; we would
welcome feedback on these.
A project of this magnitude could not have been completed without the collective
contributions of many people, some of whom we wish to acknowledge at this
juncture. First, Ken Howell deserves special thanks for his many useful suggestions.
His patience as this book production went through countless delays is also much
appreciated. Don Gwinner, Al Hicks and their production team at NREL managed to
create quality illustrations from the drawings and graphs (many in primitive form)
that were furnished to them. Maria Gamboa is thanked for very capably doing the

pre-print lay-out of the various manuscripts. Finally we offer simple thanks to our
families for their love and support and for putting up with the many weekends away
spent in putting this volume together.

Krishnan Rajeshwar
Arlington, Texas

Robert McConnell
Torrance, CA

Stuart Licht
Washington, DC

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Contents

Preface ...................................................................................................................... vii
1. Renewable Energy and the Hydrogen Economy ............................................... 1
Krishnan Rajeshwar, Robert McConnell, Kevin Harrison,
and Stuart Licht
1
Renewable Energy and the Terawatt Challenge ............................................ 1
2
Hydrogen as a Fuel of the Future .................................................................. 3
3
Solar Energy and the Hydrogen Economy .................................................. 11
4
Water Splitting and Photosynthesis ............................................................. 12

5
Completing the Loop: Fuel Cells................................................................. 14
6
Concluding Remarks ................................................................................... 16
References ................................................................................................... 16
2. The Solar Resource ............................................................................................ 19
Daryl R. Myers
1
Introduction: Basic Properties of the Sun .................................................... 19
2
The Spectral Distribution of the Sun as a Radiation Source ........................ 20
3
The Earth's Atmosphere as a Filter .............................................................. 22
4
Utilization of Solar Spectral Regions: Spectral Response of
Materials ...................................................................................................... 25
5
Reference Spectral Distributions ................................................................. 32
6
Summary ..................................................................................................... 38
References ................................................................................................... 38
3. Electrolysis of Water .......................................................................................... 41
Kevin Harrison and Johanna I vy Levene
1
Introduction ................................................................................................. 41
2
Electrolysis of Water ................................................................................... 43

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Contents

3
4
5
6
7

2.1
Alkaline .......................................................................................... 44
2.2
Proton Exchange Membrane........................................................... 45
Fundamentals of Water Electrolysis ............................................................ 50
3.1
First Principles ................................................................................ 50
3.2
Overpotentials................................................................................. 52
Commercial Electrolyzer Technologies ...................................................... 54
Electrolysis System ..................................................................................... 55
5.1
Energy Efficiency ........................................................................... 56
5.2
Electricity Costs.............................................................................. 58
Opportunities for Renewable Energy .......................................................... 59
Conclusions ................................................................................................. 60
References ................................................................................................... 61

4. A Solar Concentrator Pathway to Low-Cost Electrolytic Hydrogen ............ 65
Robert McConnell
1
Direct Conversion of Concentrated Sunlight to Electricity ......................... 65
2
The CPV Market ......................................................................................... 66
3
Higher and Higher Conversion Efficiencies ................................................ 69
4
CPV Reliability ........................................................................................... 72
5
Following in Wind Energy’s Footsteps ....................................................... 73
6
Low-Cost Hydrogen from Hybrid CPV Systems ........................................ 75
7
Describing the Hybrid CPV System ............................................................ 76
8
Discussion ................................................................................................... 81
9
Hydrogen Vision Using Hybrid Solar Concentrators .................................. 82
10 Conclusions ................................................................................................. 83
Acknowledgements ..................................................................................... 84
References ................................................................................................... 84
5. Thermochemical and Thermal/Photo Hybrid Solar Water Splitting ............ 87
Stuart Licht
1
Introduction to Solar Thermal Formation of Hydrogen............................... 87
1.1
Comparison of Solar Electrochemical, Thermal & Hybrid
Water Splitting................................................................................ 87

2
Direct Solar Thermal Water Splitting to Generate Hydrogen Fuel ............. 90
2.1
Development of Direct Solar Thermal Hydrogen ........................... 90
2.2
Theory of Direct Solar Thermal Hydrogen Generation .................. 91
2.3
Direct Solar Thermal Hydrogen Processes ..................................... 92
3
Indirect (Multi-step) Solar Thermal Water Splitting to Generate
Hydrogen Fuel ............................................................................................. 94
3.1
Historical Development of Multi-Step Thermal Processes
for Water Electrolysis .................................................................... 94
3.2
Comparison of Multi-step Indirect Solar Thermal
Hydrogen Processes ........................................................................ 96
3.3
High-Temperature, Indirect-Solar Thermal Hydrogen
Processes......................................................................................... 96

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Contents

4

5

xi

Hybrid Solar Thermal/Electrochemical/Photo (STEP) Water
Splitting ....................................................................................................... 99
4.1
Historical Development of Hybrid Thermal Processes .................. 99
4.2
Theory of Hybrid Solar Hydrogen Generation ............................... 99
4.3
Elevated Temperature Solar Hydrogen Processes and
Components .................................................................................. 111
Future Outlook and Concluding Remarks ................................................. 116
References ................................................................................................. 116

6. Molecular Approaches to Photochemical Splitting of Water ....................... 123
Frederick M. MacDonnell
1
Scope ......................................................................................................... 123
2
Fundamental Principles ............................................................................. 124
3
Nature's Photosynthetic Machinery ........................................................... 125
4
Design of Artificial Photosystems ............................................................. 129
5
The Ideal Sensitizer: Does Rubpy Come Close? ...................................... 133
5.1
Stability ....................................................................................... 133
5.2
Photophysics and Photochemistry ............................................... 136

6
Supramolecular Assemblies: Dyads, Triads and Beyond .......................... 138
6.1
Energy Transfer Quenching: Antenna Complexes ....................... 138
6.2
Bichromophores: Increasing Excited-State Lifetimes .................. 140
6.3
Reductive and Oxidative Quenching: Dyads and Triads
with Donors and Acceptors .......................................................... 142
6.4
Single versus Multi-Electron Processes ........................................ 145
7. OER and HER Co-Catalysts ...................................................................... 150
7.1
Mimicking the Oxygen Evolving Center: Water Oxidation
Catalysts ....................................................................................... 150
7.2
The Hydrogen Evolving Reaction (HER): Hydrogen
Evolution Catalysts ....................................................................... 153
8. Future Outlook and Concluding Remarks ...................................................... 154
Acknowledgements ................................................................................... 156
References ................................................................................................. 156
7. Hydrogen Generation from Irradiated Semiconductor-Liquid
Interfaces .......................................................................................................... 167
Krishnan Rajeshwar
1
Introduction and Scope .............................................................................. 167
2
Types of Approaches ................................................................................. 170
3
More on Nomenclature and the Water Splitting Reaction

Requirements ............................................................................................. 172
4
Efficiency of Photoelectrolysis .................................................................. 178
5
Theoretical Aspects ................................................................................... 180
6
Oxide Semiconductors............................................................................... 183
6.1
Titanium Dioxide: Early Work .................................................... 183

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Contents

6.2

7
8
9
10
11
12
13
14
15

Studies on the Mechanistic Aspects of Processes at the

TiO2-Solution Interface ................................................................ 186
6.3
Visible Light Sensitization of TiO2............................................... 186
6.4
Recent Work on TiO2 on Photosplitting of Water or on the
Oxygen Evolution Reaction ......................................................... 187
6.5
Other Binary Oxides .................................................................... 190
6.6
Perovskite Titanates and Related Oxides..................................... 192
6.7
Tantalates and Niobates ................................................................ 197
6.8
Miscellaneous Multinary Oxides .................................................. 198
Nitrides, Oxynitrides and Oxysulfides ...................................................... 200
Metal Chalcogenide Semiconductors ........................................................ 202
8.1
Cadmium Sulfide .......................................................................... 202
8.2
Other Metal Chalcogenides .......................................................... 204
Group III-V Compound Semiconductors .................................................. 205
Germanium and Silicon ............................................................................. 206
Silver Halides ............................................................................................ 208
Semiconductor Alloys and Mixed Oxides ................................................. 208
12.1 Semiconductor Composites .......................................................... 208
Photochemical Diodes and Twin-Photosystem Configurations for
Water Splitting .......................................................................................... 210
Other Miscellaneous Approaches and Hydrogen Generation from
Media Other than Water ............................................................................ 211
Concluding Remarks ................................................................................. 213

Acknowledgments ..................................................................................... 213
References ................................................................................................. 213

8. Photobiological Methods of Renewable Hydrogen Production .................... 229
Maria L. Ghirardi, Pin Ching Maness, and Michael Seibert
1
Introduction ............................................................................................... 229
2
Green Algae............................................................................................... 230
2.1
Mechanism of Hydrogen Production .................................. ..........230
2.2
Hydrogenase-Catalyzed H2 Production........................................ 233
2.3
[FeFe]–hydrogenases. ................................................................... 234
3
Cyanobacteria ............................................................................................ 235
3.1
Mechanisms of Hydrogen Production ......................................... 235
3.2
Hydrogenase-Catalyzed H2 Production ....................................... 236
3.3
[NiFe]-Hydrogenases.................................................................... 238
3.4
Nitrogenase-Catalyzed H2 Production ......................................... 240
3.5
Nitrogenases ................................................................................. 241
4. Other Systems............................................................................................ 242
4.1
Non-Oxygenic Purple, Non-Sulfur Photosynthetic Bacteria ........ 242

4.2
Mixed Light/Dark Systems .......................................................... 243
4.3
Bio-Inspired Systems .................................................................... 244
5
Scientific and Technical Issues.................................................................. 245
5.1
General ......................................................................................... 245
5.2
Oxygen Sensitivity of [FeFe]-Hydrogenases ................................ 246

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5.3
5.4

6

Oxygen Sensitivity of [NiFe]-Hydrogenases ................................ 248
Competition between Different Pathways for
Photosynthetic Reductants ........................................................... 249
5.5
Down-Regulation of Electron Transport Rates............................. 250
5.6
Low-Light Saturation Properties of Photosynthetic

Organisms .................................................................................... 251
5.7
Photobioreactor and System Costs ............................................... 252
5.8
Genomics Approaches. ................................................................. 254
Future Directions ....................................................................................... 254
Acknowledgments ..................................................................................... 255
References ................................................................................................. 255

9. Centralized Production of Hydrogen using a Coupled Water
Electrolyzer-Solar Photovoltaic System ......................................................... 273
James Mason and Ken Zweibel
1
Introduction ............................................................................................... 273
2
Description of a PV Electrolytic H2 Production and Distribution
System ....................................................................................................... 274
3
Capital Investment and Levelized Price Estimates .................................... 281
4
Sensitivity Analysis: H2 Production and PV Electricity Prices ................. 285
5
Economic Analysis of Second Generation (Year 31–Year 60) H2
Systems...................................................................................................... 289
6
Life Cycle Energy and GHG Emissions Analyses .................................... 294
6.1
Life Cycle Analysis Methods ....................................................... 294
6.2
Life Cycle Energy and GHG Emissions Analyses Results ........... 296

7
System Energy Flow/Mass/Balance Analysis ........................................... 296
8
Conclusions: Summary of Results and Suggestions for Future
Analysis. .................................................................................................... 298
Appendices ................................................................................................ 305
1
Energy Units and CO2 Equivalent Emissions Estimates............... 305
2
Levelized Price Estimates Derived by Net Present Value
Cash Flow Analysis ...................................................................... 305
3
Adiabatic Compression Formula ................................................. 307
4
Deviations from DOE H2A Assumptions ................................... 308
5
Summary of Reviewer Comments with Responses ...................... 309
References ................................................................................................. 312
Index ....................................................................................................................... 315

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Contributors

Maria L. Ghiradi,
National Renewable Energy Laboratory
1617 Cole Blvd., Golden, CO 80401

Kevin Harrison

National Renewable Energy Laboratory
NREL MS3911
1617 Cole Blvd., Golden, CO 80401
Ph: 303-384-7091, F:303-384-7055,
Johanna Ivy Levene
National Renewable Energy Laboratory
NREL MS3911
1617 Cole Blvd., Golden, CO 80401

Stuart Licht
Chemistry Division
National Science Foundation
4201 Wilson Blvd., Arlington, VA 022230
Ph: 703-292-4952,
Chemistry Department
100 Morrissey Boulevard
University of Massachusetts, Boston, MA 02135-3395
Ph: 617-287-6156,

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xvi

Contributors

Frederick M. MacDonnell
Department of Chemistry and Biochemistry
The University of Texas at Arlington, Arlington, TX 76019-0065
Ph: 817-272-2972, F:817-272-3808,

Pin Ching Maness
National Renewable Energy Laboratory
1617 Cole Blvd., Golden, CO 80401

James Mason
Hydrogen Research Institute
52 Columbia St., Farmingdale, NY 11735
Ph: 516-694-0759, E:
Robert McConnell
Amonix, Inc.
3425 Fujita St., Torrance, CA 90505
Ph: 310-325-8091, F: 310-325-0771, E:
Daryl Myers
Electric System Center
NREL MS3411
1617 Cole Blvd., Golden, CO 80401
Ph: 303-384-6768, F:303-384-6391, E: ,
W: />Krishnan Rajeshwar
College of Science, Box 19065
The University of Texas at Arlington, Arlington, TX 76019
Ph: 817-272-3492, F:817-272-3511, E: ,
Michael Seibert
National Renewable Energy Laboratory
1617 Cole Blvd., Golden, CO 80401
Ph: 303-384-6279, F: 303-384-6150,
Ken Zweibel
Primestar Solar Co., Longmont, CO


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Biographical Sketches of Authors

Maria L. Ghirardi is a Senior Scientist at NREL and a Research Associate
Professor at the Colorado School of Mines. She has a B.S., an M.S. and a Ph.D
degree in Comparative Biochemistry from the University of California at Berkeley
and has extensive experience working with photosynthetic organisms. Her research at
NREL involves photobiological H2 production and covers metabolic, biochemical and
genetic aspects of algal metabolism, generating over 60 articles and several patents.
Kevin W. Harrison is a Senior Engineer in the Electrical Systems Center at NREL.
He received his Ph.D. at the University of North Dakota and leveraging management,
automated equipment design and quality control experience, gained while working
for Xerox Corporation, he joined NREL in 2006. At NREL he leads all aspects of the
renewable hydrogen production task whose objective is to improve the efficiency and
reduce the capital costs of a closely coupled wind to hydrogen demonstration project.
Generally speaking his research interests are in reducing the environmental impact of
the world’s energy use by integrating and utilizing renewable energy for electricity
and transportation fuels.
Johanna Ivy Levene is a Senior Chemical Applications Engineer at NREL. She
specializes in the technical and economic analysis of electrolysis systems, and her
current focus is the production of fuels from renewable resources. Prior to her work
at NREL, Johanna has worked as a process control engineer, a database
administrator, a systems administrator and a programmer. Results from her work
have been published in Solar Today and Science.
Stuart Licht is a Program Director in the Chemistry Division of the National
Science Foundation (NSF) and Professor of Chemistry at the University of
Massachusetts, Boston. His research interests include solar and hydrogen energy,
energy storage, unusual analytical methodologies, and fundamental physical
chemistry. Prof. Licht received his doctorate in 1986 from the Weizmann Institute of

Science, followed by a Postdoctoral Fellowship at MIT. In 1988 he became the first
Carlson Professor of Chemistry at Clark University, and in 1995 a Gustella

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xviii

Biographical Sketches of Authors

Professor at the Technion Israel Institute of Science, in 2003 became Chair of the
Department of Chemistry at the University of Massachusetts Boston, and in 2007 a
Program Director at the NSF. He has contributed 270 peer reviewed papers and
patents ranging from novel efficient solar semiconductor/electrochemical processes,
to unusual batteries, to elucidation of complex equilibria and quantum electron
correlation theory.
F. M. MacDonnell is Professor of Inorganic Chemistry at the University of Texas at
Arlington (UTA). He received his PhD at Northwestern University in 1993. After a
postdoctoral stint at the Chemistry Department of Harvard University, he joined the
Chemistry and Biochemistry Department at UTA in 1995. His research interests are
in the design of photocatalysts for light harvesting and energy conversion and has
published over 100 articles in these areas.
Pin Ching Maness is a Senior Scientist at NREL. She received her Masters Degree
in 1976 at Indiana State University, Terre Haute, IN. She worked as a Research
Specialist at the University of California, Berkeley, CA from 1976 to 1980, before
joining NREL in 1981. Her research interests are in studies of the physiology,
biochemistry, and molecular biology of various biological H2-production reactions in
cyanobacteria, photosynthetic bacteria, and cellulolytic fermentative bacteria.
James M. Mason is Director of the Hydrogen Research Institute in Farmingdale,
New York. He received his PhD at Cornell University in 1996. His research interests

are the economic modelling of centralized hydrogen production and distribution
systems using renewable energy sources.
Robert D. McConnell recently joined Amonix, Inc., a concentrator photovolatics
(PV) company located in Torrance, CA as Director of Government Affairs and
Contracts. He earned his PhD at Rutgers University in Solid State Physics following
a Bachelor’s degree in Physics at Reed College in Portland, Oregon. After a
postdoctoral stint at the University of Montreal and employment at the research
institute of the electric utility, Hydro Quebec, he joined NREL in 1978. He has
authored numerous papers and edited or co-edited five books and chaired four
international conferences on centrator PV. His technology interests include
concentrator PV, future generation PV concepts, hydrogen, superconductivity, and
wind energy. He has served as Chairman of the Energy Technology Division of the
Electrochemical Society and is presently Convener of the international working
group developing concentrator PV standards under the aegis of the International
Electrotechnical Commission located in Geneva, Switzerland.
Daryl R. Myers is a Senior Scientist at NREl. In 1970 He received a Bachelor of
Science in Applied Mathematics from the University of Colorado, Boulder, School of
Engineering. Prior to joining NREL in 1978, he worked for four years at the
Smithsonian Institution Radiation Biology Laboratory in Rockville Maryland, and is
a Cold War veteran, serving as a Russian linguist in the United States Army from
1970 to 1974. He has over 32 years of experience in terrestrial broadband and
spectral solar radiation physics, measurement instrumentation, metrology

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Biographical Sketches of Authors

xix

(calibration), and modelling radiative transfer through the atmosphere. Daryl is active
in International Lighting Commission (CIE) Division 2 on Physical Measurement of
Light and Radiation, the American Society for Testing and Materials (ASTM)
committees E44 on Solar, Geothermal, and Other Alternative Energy Sources and
G03 on Weathering and Durability, and the Council for Optical Radiation
Measurements (CORM).
Krishnan Rajeshwar is a Distinguished Professor in the Department of Chemistry
and Biochemistry and Associate Dean in the College of Science at the University of
Texas at Arlington. He is the author of over 450 refereed publications, several invited
reviews, book chapters, a monograph, and has edited books, special issues of
journals, and conference proceedings in the areas of materials chemistry, solar energy
conversion, and environmental electrochemistry. Dr. Rajeshwar is the Editor of the
Electrochemical Society Interface magazine and is on the Editorial Advisory Board
of the Journal of Applied Electrochemistry. Dr. Rajeshwar has won many Society
and University awards and is a Fellow of the Electrochemical Society.
Michael Seibert is a Fellow at the National Renewable Energy Laboratory in
Golden, CO, USA. He received his Ph.D. in the Johnson Research Foundation at the
University of Pennsylvania and then worked at GTE Laboratories before joining
NREL (formerly the Solar Energy Research Institute) in 1977. His research has
resulted in over 180 publications and several patents in the areas of materials
development for electronic microcircuits, primary processes of bacterial and plant
photosynthesis, cryopreservation and photomorphogenesis of plant tissue culture,
water oxidation by photosystem II in plants and algae, microbial H2 production,
hydrogenase structure and function, genomics of Chlamydomonas, and
computational approaches for improving H2 metabolism in algae. He also holds a
concurrent position as Research Professor at the Colorado School of Mines and is a
Fellow of the AAAS.
Ken Zweibel is President of PrimeStar Solar, a CdTe PV company located in
Colorado, USA. He graduated in Physics from the University of Chicago in 1970. He
was employed for 27 years at SERI and then NREL in Golden, CO, where he worked

on the development of CdTe, copper indium diselenide, and amorphous and thin film
silicon. When he left in December 2006, he was manager of the Thin Film PV
Partnership Program. He has published numerous papers and articles, and two books
on PV, the most recent being, “Harnessing Solar Power: The PV Challenge.” Besides
the success of PrimeStar Solar, he is interested in solar policy and solutions to
climate change and rising energy prices.

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1
Renewable Energy and the Hydrogen Economy

Krishnan Rajeshwar,1 Robert McConnell,2 Kevin Harrison,2 and Stuart Licht3
1

University of Texas at Arlington, Arlington, TX
NREL, Golden, CO
3
University of Massachussetts, Boston, MA
2

1 Renewable Energy and the Terawatt Challenge
Technological advancement and a growing world economy during the past few decades have led to major improvements in the living conditions of people in the developed world. However, these improvements have come at a steep environmental
price. Air quality concerns and global climate impact constitute two major problems
with our reliance on fossil energy sources. Global warming as a result of the accumulation of greenhouse gases such as CO2 is not a new concept. More than a century ago, Arrhenius put forth the idea that CO2 from fossil fuel combustion could
cause the earth to warm as the infrared opacity of its atmosphere continued to rise.1
The links between fossil fuel burning, climate change, and environmental impacts
are becoming better understood.2 Atmospheric CO2 has increased from ~275 ppm to
~370 ppm (Figure 1); unchecked, it will pass 550 ppm this century. Climate models

indicate that 550 ppm CO2 accumulation, if sustained, could eventually produce
global warming comparable in magnitude but opposite in sign to the global cooling
of the last Ice Age.3 The consequences of this lurking time bomb could be unpredictably catastrophic and disastrous as recent hurricanes and tsunamis indicate.
Every year, a larger percentage of the 6.5 billion global population seeks to improve their standard of living by burning ever-increasing quantities of carbon-rich
fossil fuels. Based on United Nations forecasts, another 2.5 billion people are expected by 2050 with the preponderance of them residing in poor countries.4 Coupled
with this growing population’s desire to improve their quality of life are the developed countries already high and rising per capita energy use which promises to add
to the environmental pressure.
Oil, coal, and natural gas have powered cars, trucks, power plants, and factories,
causing a relatively recent and dramatic buildup of greenhouse gases in the atmos-

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2

Krishnan Rajeshwar et al.

Fig. 1. Atmospheric carbon dioxide record from Mauna Loa. Data courtesy of C. D. Keeling
and T. P. Whorf.

phere, most notably CO2. The anthropogenic buildup of heat-trapping gases is intensifying the earth’s natural greenhouse effect, causing average global temperatures to
rise at an increasing rate. We appear to be entering into a period of abrupt swings in
climate partially due to buildup of human-released CO2 in the atmosphere. Most
alarming is not the fact that the climate is changing but rather the rate at which the
buildup of CO2 is occurring.
Ice core samples from Vostok, Antarctic, look back over 400,000 years before
present at atmospheric CO2 levels by examining the composition of air bubbles
trapped in the polar ice buried over 3623 m (11,886 ft) deep.5 These data show that
the range of CO2 concentrations over this time period have been relatively stable,
cycling between about 180 and 300 parts per million by volume (ppmv). According

to the World Meteorological Organization the CO2 concentration in 2005 reached an
unprecedented 379.1 ppmv.6
This environmental imperative requires us to quickly come to terms with the actual costs, including environmental externalities, of all of our energy use. Only then
will the economic reality of energy consumption be realized and renewable sources
expand through true market forces. That is not to say that fossil fuels like oil, natural
gas, and coal do not have a future in helping to meet this growing demand. However, it should go without saying that all new sources of CO2 should be captured and
stored (i.e., sequestered). Although integrating the systems required to safely and
economically storing CO2 deep underground have not been realized. More than
ever, CO2 released into the atmosphere by coal-fired power plants must be addressed

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Renewable Energy and the Hydrogen Economy

3

to effectively deal with global climate change. In addition to greenhouse gas emissions, destructive extraction and processing of the fuel, fine particulates of 2.5 micrometers (μm) released from coal-fired power plants are responsible for the deaths
of roughly 30,000 Americans every year.7
Even notwithstanding this climate change and global warming concern are issues
with the supply side of a fossil-derived energy economy. Gasoline and natural gas
supplies will be under increasing stress as the economies of heavily-populated developing countries (such as India and China) heat up and become more energy intensive. It is pertinent to note that this supply problem is exacerbated because the United States alone consumes a disproportionately higher fraction (more than the next
five highest energy-consuming nations, Ref. 8) of the available fossil fuel supply.
There are no signs that the insatiable energy appetite of the U. S. and other advanced
parts of the world are beginning to wane. While there is considerable debate about
when global oil and natural gas production is likely to peak,9 there is no debate that
fossil fuels constitute a non-renewable, finite resource. We are already seeing a
trend in some parts of the world (e.g., Alberta, Canada) of a switch to “dirtier” fossil
fuels, namely, coal, heavy oil or tar sand as petroleum substitutes. This switch
would mean an increase in CO2 emissions (note that the carbon content of these

sources is higher than gasoline or natural gas), a greater temperature rise than is now
being forecast, and even more devastating effects on the earth’s biosphere than have
already been envisioned.10
Currently, renewable energy only constitutes a very small fraction of the total
energy mix in the U. S. and in other parts of the world (Figure 2). For example, in
2000, only about 6.6 quads (one quad is about 1018 J) of the primary energy in the U.
S. came from renewables out of a total of 98.5 quads.11 Of this small fraction supplied by renewable energy, about 3.3 quads were from biomass, 2.8 from hydroelectric generation, 0.32 from geothermal sources, 0.07 from solar thermal energy and
0.05 quads from wind turbines.8 This profile would have to switch to an energy mix
that resembles the right-side panel in Figure 2 if the CO2 emissions are to be capped
at environmentally safe levels. This is what the late Professor Rick Smalley, winner
of the Nobel Prize in Chemistry, referred to as the Terawatt Challenge. Recent analyses12 have posited that researching, developing, and commercializing carbon-free
primary power to the required level of 10-30 TW (one terawatt = 1012 W) by 2050
will require efforts of the urgency and scale of the Manhattan Project and the Apollo
Space Program.
This book examines the salient aspects of a hydrogen economy, particularly within the context of a renewable, sustainable energy system.

2 Hydrogen as a Fuel of the Future
Jules Verne appears to be one of the earliest people to recognize, or at least articulate, the idea of splitting water to produce hydrogen (H2) and oxygen (O2) in order to
satisfy the energy requirements of society. As early as 1874 in The Mysterious Island, Jules Verne alluded to clean hydrogen fuels, writing:

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Krishnan Rajeshwar et al.

Fig. 2. The terawatt renewable energy challenge; the energy mix has to switch from the panel
on the left to the panel on the right to cap CO2 levels at safe limits. Data from the International Energy Agency.

"Yes, my friends, I believe that water will someday be employed as fuel, that
hydrogen and oxygen, which constitute it, used singly or together, will furnish
an inexhaustible source of heat and light….I believe, then, that when the deposits of coal are exhausted, we shall heat and warm ourselves with water.
Water will be the coal of the future."
Remarkable words indeed from a prophetic visionary who foresaw also the technological development of spacecraft and submarines. Hydrogen gas was first isolated by Henry Cavendish in 1766 and later recognized as a constituent of water by
Lavoisier in 1783.13 The production of hydrogen and oxygen by the electrolytic decomposition of water has been practiced since the year 1800, when the process was
first discovered by Nicholson and Carlisle.14 Since then, the idea of society using
hydrogen as a primary energy carrier has been explored and refined.
In the late 1920s and the early 1930s a German inventor, Rudolf A. Erren, recognized and worked towards producing hydrogen from off-peak electricity and modifying the internal combustion engine to run on hydrogen.15 Erren’s primary objective
was to eliminate pollution from the automobile and reduce oil imports. In the 1970s
Derek Gregory appears to have been one of the leading advocates in creating the
case for a hydrogen-based economy.13,15,16
The literature suggests that the term hydrogen economy may have been coined by
H. R. Linden, one of Gregory’s colleagues at the Institute of Gas Technology, in
1971.13 Gregory points to hydrogen’s environmental benefits and recognizes that,
while fossil fuels are inexpensive, requiring the atmosphere to assimilate the byproducts of their combustion is not without consequence.

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Renewable Energy and the Hydrogen Economy

5

The water electrolyzer industry grew substantially during the 1920s and 1930s, as
elaborated later in Chapter 3. This included products from companies such as Oerlikon, Norsk Hydro, and Cominco in multi-megawatt sizes.14,17,18 Most of these installations were near hydroelectric plants that supplied an inexpensive source of electricity. As more hydrogen was needed for industries, steam reforming of methane
gradually took over as the hydrogen production process of choice because it was less
expensive.
Hydrogen is often blamed for the 1937 Hindenburg disaster. The shell of the
German airship was a mixture of two major components of rocket fuel, aluminum

and iron oxide, and a doping solution which was stretched to waterproof the outer
hull. Researchers concluded that the coating of the Hindenburg airship was ignited
by an electrical discharge and the ensuing explosion to be inconsistent with a hydrogen fire.19 It turns out that 35 of the 37 people who died in the disaster, perished from
jumping or falling from the airship to the ground. Only two of the victims died of
burns, and these were from the burning airship coating and on-board diesel fuel.20
Modern laboratory tests confirmed that the 1930s fabric samples to still be combustible.
“Although the benefits of the hydrogen economy are still years away, our biggest challenges from a sustainability standpoint are here today,”
said Mike Nicklas, Past Chair of the American Solar Energy Society, during his
opening comments at the first Renewable Hydrogen Forum in Washington, D.C., in
April 2003.21
Hydrogen (H) is the simplest of atoms, consisting of one proton and one electron
also called a protium. As atoms, hydrogen is very reactive and prefers to join into
molecular pairs (H2) and when mixed in sufficient quantities with an oxidant (i.e.,
air, O2, Cl, F, N2O4, etc.) becomes a combustible mixture. Like all other fuels, H2
requires proper understanding and handling to avoid unwanted flammable or explosive environments. Hydrogen is not a primary source of energy; rather it is an energy carrier much like electricity. Therefore, energy is required to extract hydrogen
from substances like natural gas, water, coal, or any other hydrocarbon.
At 25 °C and atmospheric pressure the density of air is 1.225 kg m-3 while hydrogen is 0.0838 kg m-3, making it 14.6 times lighter than air. This is an important safety
consideration in that a hydrogen leak will dissipate quickly. Hydrogen’s positive
buoyancy significantly limits the horizontal spreading of hydrogen that could lead to
combustible mixtures. Hydrogen is the lightest (molecular weight 2.016) and smallest of all gases requiring special considerations for containing and sensing a leak.
Figure 3 shows the two types of molecular hydrogen distinguished by the spin,
ortho- and para-hydrogen. They differ in the magnetic interactions as ortho-hydrogen
atoms are both spinning in the same direction and in para-hydrogen the protons are
spinning anti-parallel. At 300 K, the majority (75%) is ortho-hydrogen, while at 20
K 99.8% of the hydrogen molecules are para-hydrogen. As the gas transitions from
gas to liquid at 20 K heat is released and ortho-hydrogen becomes unstable.22 Hydrogen becomes a liquid below its boiling point of −253 °C (20 K) at atmospheric pres-

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Krishnan Rajeshwar et al.

Fig. 3. Ortho- (left) and para-hydrogen (right).

sure. Pressurization of the hydrogen to 195 pisg (13 barg) increases the boiling point
to −240 °C (−400 °F), pressures above that don’t return a significant improvement.22
At ambient temperature and pressure hydrogen is colorless, odorless, tasteless
and nontoxic. However, leaks of hydrogen (or any gas for that matter) can displace
oxygen and act as an asphyxiant. Any atmosphere with less than 19.5% oxygen by
volume in considered oxygen deficient and asphyxiation can lead to physiological
hazards.
The primary hazard associated with gaseous hydrogen is the unintentional mixing
of the fuel with an oxidant (typically air) in the presence of an ignition source. Hydrogen fires and deflagrations have resulted when concentrations within the flammability limit were ignited by seemingly harmless ignition sources. Ignition sources
include electrical, mechanical, thermal and chemical. For example; sparks from
valves, electrostatic discharges, sparks from electrical equipment, mechanical impact, welding and cutting, open flame, personnel smoking, catalyst particles and
lightning strikes in the proximity of hydrogen vent stacks.23
With the exception of helium, hydrogen has the lowest boiling point at atmospheric pressure of where it becomes a transparent and odorless liquid. Liquid hydrogen has a specific gravity of 0.071, which is roughly 1/14th the density of water and
is neither corrosive nor reactive. The low specific gravity of liquid hydrogen further
reveals hydrogen’s low volumetric energy density in that a cubic meter of water
contains more hydrogen (111 kg) than a cubic meter of pure hydrogen in liquid state
(71 kg). The values of the main physical properties of gaseous hydrogen are shown
in Table 1.
Leaking hydrogen gas and (once ignited) its flame are nearly invisible. The pale
blue flame of a hydrogen fire is barely visible and is often detected by placing a
standard household wicker broom in the path of the suspected hydrogen flame. The
hydrogen flame temperature in air (2045 C, 3713 F) releases most of its energy in the
ultraviolet (UV) region requiring UV sensors for detecting the presence of a flare or
fire. The UV radiation from a flaring hydrogen fire can also cause burns akin to

over-exposure to the sun’s damaging UV radiation.

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Renewable Energy and the Hydrogen Economy

7

Table 1. Selected properties of gaseous hydrogen at 20 °C and 1 atm.
Physical Property

Units

Molecular weight
Density
Specific gravity

2.016
0.0838
0.0696

kg/m3
(Air = 1)

Viscosity
Diffusivity
Thermal conductivity

8.813 x 10-5

1.697
0.1825

g/cm sec
m2/hr
W/m K

Expansion ratio
Boiling point (1 atm)
Specific heat, constant pressure

1:848
-253 (-423)
14.29

Specific heat, constant volume
Specific volume
Diffusion coefficient in air

10.16
11.93
6.10

J/g K
m3/kg
cm2/sec

4098
64.44

kJ/kg
J/g K

Enthalpy
Entropy

Liquid to gas
°C (°F)
J/g K

The amount of thermal radiation (heat) emitted from a hydrogen flame is low and
is hard to detect by feeling (low emissivity). Most commercially available combustible gas detectors can be calibrated for hydrogen detection. Typically alarms from
these sensors are set by the manufacturer between 10%–50% of the lower flammability limit (LFL) of hydrogen to avoid the presence of an unwanted flammable environment.
Table 2 compares the same fuels as above and reports their volumetric energy
density in kg m-3. Hydrogen has the highest energy content per unit mass than any
fuel making it especially valuable when traveling into space. As mentioned earlier,
hydrogen suffers volumetrically when compared with traditional fuels making storing sufficient on-board terrestrial vehicles an engineering challenge.
The LFL of hydrogen represents the minimum concentration required below
which the mixture is too lean to support combustion.24 Hydrogen has a wide flammability range of (4%–75%) while gasoline is (1.5%–7%) when mixed with air at
standard temperature (25 ºC) and pressure (1 atm). Hydrogen in oxygen has a
slightly wider flammability range (4%–95%). Table 3 summarizes a selected number of important combustion properties of hydrogen.

Table 2. Comparing hydrogen properties with other fuels. Based on LHV and 1 atm,
25 °C for gases.
Hydrogen

Methane

-3

Gasoline

Diesel

Density, kg m
0.0838
0.71
702
855
Energy density, MJ m-3
10.8
32.6
31,240
36,340
Energy density, kWh m-3
3.0
9.1
8680
10,090
33.3
12.8
12.4
11.8
Energy, kWh kg-1
*Energy density = LHV ∗ density ( ), and the conversion factor is 1 kWh = 3.6 MJ.

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Methanol
799

14,500
4030
5.0


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Krishnan Rajeshwar et al.
Table 3. Selected combustion properties of hydrogen at 20 oC and 1 atm.a
Combustion Property

Units

Flammability limits in air
Flammability limits in oxygen
Detonability limits in air

4 – 75
4 – 95
18 – 59

Detonability limits in oxygen
Minimum ignition energy in air
Auto ignition temperature

15 – 90
17
585 (1085)

vol%

J
°C (°F)

0.064
0.061
2.7 – 3.5

cm
cm2/sec
m/s

0.1
2045 (3713)

°C (°F)

Quenching gap in air
Diffusion coefficient in air
Flame velocity
Flame emissivity
Flame temperature
a
From Ref. 19.

vol%
vol%
vol%

Each fuel is limited to a fixed amount of energy it can release when it reacts with
an oxidant. Every fuel has been experimentally tested to determine the amount of

energy it can release and is reported as the fuel’s higher heating value (HHV) and
lower heating value (LHV). The difference between the two values is the latent heat
of vaporization of water, and the LHV assumes this energy is not recovered.22 In
other words, LHVs neglect the energy in the water vapor formed by the combustion
of hydrogen in the fuel because it may be impractical to recover the energy released
when water condenses. This heat of vaporization typically represents about 10% of
the energy content.
It is often confusing to know which heating value to use when dealing with similar processes such as electrolysis and fuel cells. The appropriate heating value depends on the phase of the water in the reaction products. When water is in liquid
form, the HHV is used; if water vapor (or steam) is formed in the reaction, then the
LHV would be appropriate. An important distinction is that water is produced in the
form of vapor in a fuel cell as well as in a combustion reaction and, therefore, the
LHV represents the amount of energy available to do work. Table 4 shows both the
LHV and the HHV for common fuels.
Obviously, the most important virtue of using hydrogen as a fuel is its pollutionfree nature. When burned in air, the main combustion product is water with O2 in a
fuel cell to directly produce electricity; the only emission is water vapor. Indeed this
Table 4. HHVs and LHVs at 25 °C and 1 atm of common fuels, kJ g-1 a
Fuel
Hydrogen
Methane
Gasoline
Diesel
Methanol
a
From Ref. 22.

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HHV

LHV

141.9
55.5
47.5
44.8
20.0

119.9
50.0
44.5
42.5
18.1